Now you may find that more and more children arc becoming very heavy．Why? Maybe there're three main ____ ．

    Firstly，there are too ____ shops around schools．The shops sell unhealthy foods，such as fried chicken and ice cream at low prices．So some children don't like to eat at home．If there are fewer of these shops，then probably children will ____ less unhealthy food．

    Secondly，children have ____ eating habits．They like fast food．Today we can see McDonald's and KFC everywhere．Many children like fast food very much．In fact．most of the fast food is unhealthy ____ it has too much salt（盐），sugar and fat(脂肪)．

    Thirdly，these days children ____ little exercise．When they get home，they sit before the televisions or their computers and play computer games．It's not a good habit，and it also gives them time to eat more unhealthy food．They should go ____ and play sports or games．

    For children's health，parents should ask children to do more sports and eat ____fast food．

青蛙前后肢的主要作用是____。